Field of the Disclosure
Embodiments generally relate to input sensing and, in particular, to calibrating charge mismatch in a baseline correction circuit.
Description of the Related Art
Input devices including proximity sensor devices (also commonly called touchpads or touch sensor devices) are widely used in a variety of electronic systems. A proximity sensor device typically includes a sensing region, often demarked by a surface, in which the proximity sensor device determines the presence, location, and/or motion of one or more input objects. Proximity sensor devices may be used to provide interfaces for the electronic system. For example, proximity sensor devices are often used as input devices for larger computing systems (such as opaque touchpads integrated in, or peripheral to, notebook or desktop computers). Proximity sensor devices are also often used in smaller computing systems (such as touch screens integrated in cellular phones).
Such input devices include sensor electrodes that are driven with a signal for capacitive sensing. Processing circuitry processes the signal received from the sensor electrodes in order to determine capacitance of the sensor electrodes. A change in capacitance of the sensor electrodes indicates presence or absence of an input object proximate to a sensor electrode as well as location of the input object.
Sensor electrodes form “capacitive pixels” with each other or with other elements such as input objects. Each capacitive pixel has an associated baseline measurement, which is a measurement from the capacitive pixel that a processing system measures with no input object in the sensing region. An input device performs baselining operations to account for variations or defects in the input device that may cause these baseline measurements to deviate from what is desired.
Baseline correction is usually achieved by adding or subtracting a certain amount of the charge received by processing circuitry when an associated sensor electrode is driven for capacitive sensing, such that an expected nominal output is obtained from the processing circuitry. One type of baseline operation, a current-mode baseline operation, adjusts the amount of current over a certain duration of time. Because of variations in manufacturing processes related to the baseline circuitry, current-mode baseline operations may introduce a “common mode voltage offset” into a signal associated with the processing circuitry. A common mode voltage offset is a difference between the average of the output signal over a sensing cycle and a value that is deemed to be a “zeroed” value (such as a mid-rail voltage). This common mode voltage offset results in a reduced dynamic range of the processing circuitry, which may reduce the range of input signals over which the input device may operate, thus reducing the ability to sense the presence of an input object.